The Asus R556UB Laptop Supports Single and Dual Drive Configurations. (One 2.5" Solid State Drive or Hard Drive in the Internal Primary Bay and One 2.5" Solid State Drive or Hard Drive in the Modular Bay) Note: The second 2.5" Drive replaces the optical drive when installed. If installing the second 2.5" Drive in replace of the optical drive, you will need the Modular Bay p/n 04150-00030000 for installation.